1. Field of the Invention
This invention belongs to the field of processes for preparing diphenyls and, more particularly, to a process for peparing 4,4'-dihydroxydiphenyl ether from the acid cleavage of a hydroperoxide intermediate.
2. Description of the Prior Art
The diphenyl ether,4-4'-dihydroxydiphenyl ether, (DHDPE) which is represented by the structural formula ##STR1## bears a close resemblance to the widely used diphenyl compound, bisphenol A (4,4'-isopropylidenebisphenol), which is represented by the structural formula ##STR2## differing from the latter only in the presence of a bridging ether oxygen in place of an isopropylene bridge.
DHDPE, a known compound, despite its attractive properties for numerous applications, has yet to be synthesized in an economical manner and to date has not been commercially manufactured.
An economical synthesis of DHDPE is desirable because it would make commercially available a monomer which confers an unusually high degree of thermal and oxidative stability to compounds and polymers prepared therefrom. Like the extensively employed bisphenol A, DHDPE is useful for preparing polyesters, polyurethanes, polycarbonates, polyethers, mixed polyester/polysulfones and epoxy and phenolic resins. However, unlike bisphenol A, DHDPE conveys to its polymers the properties of a stable aromatic ether. Unlike bisphenol A, there are no deleterious pendant alkyl groups in the structure of DHDPE. In addition to stability, DHDPE convers on its oligomers and polymers a high degree of internal pasticization, an inherent property of polyaromatic ethers. Its oligomers are valuable for extreme pressure lubricants and transformer fluids.
Valuable reactive flame retardant compounds can be made from DHDPE. When fully brominated, the product octabromodihydroxydiphenyl ether can be incorporated as an integral unit of a polymer, in contrast to the unreactive halogenated diphenyl ethers which are at present widely used as flame retardants. Like the very popular decabromodiphenyl ether, DHDPE is substantially nontoxic when incorporated into a polymer.
The brominated derivatives of bisphenol A are reactive flame retardants currently used in high volume. A disadvantage of these is the oxidizable, hence flammable, isopropylidene groups. DHDPE when fully brominated presents only an ether oxygen which enhances flame retardancy properties and contains more bromine than fully brominated bisphenol A. Brominated DHDPE can be converted to nonreactive flame retardants by proper etherification or esterification.
DHDPE was first prepared by the tedious and expensive method of dinitration of the para positions of diphenyl ether, reduction to the corresponding diamine and diazotization and decomposition of the latter with methanol. U.S. Pat. No. 3,290,386 discloses the hydrolysis of 4,4'-dibromodiphenyl ether by excess aqueous caustic in the presence of catalytic quantities of alkaline peroxide and cuprous halide to provide DHDPE. While this procedure is an improvement over the DHDPE synthesis described in German Pat. No. 609,080 which employs methanolic caustic in the presence of copper powder to effect hydrolysis, it has nevertheless not become a commercial reality. Other processes for preparing DHDPE which also have not met with any significant measure of success are described in U.S. Pat. No. 2,739,171 (catalytic dehydration of dihydroxy hydroquinone) and U.S. Pat. No. 3,886,218 (condensation of hydroquinone in an inert organic solvent in the presence of an aluminum silicate catalyst).